Electronic circuit for altering the slope of ramp signals produced during the time intervals between recurring pairs of pulses, the slope being altered in accordance with the time difference between a previous time interval, and the duration of a corresponding ramp signal



March 1965 R. A. HACKBORN ETAL 3,175,161

ELECTRONIC CIRCUIT FOR ALTERING THE SLOPE OF RAMP SIGNALS PRODUCED DURING THE TIME INTERVALS BETWEEN RECURRING PAIRS OF PULSE-S, THE SLOPE BEING ALTERED IN ACCQRDANCE WITH THE TIME DIFFERENCE BETWEEN A PREVIOUS TIME INTERVAL, AND THE DURATION OF A CORRESPONDING RAMP SIGNAL Filed Jan. 24, 1965 Cl. "I I g Ii I e3-- I: I k ,2:

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,5 L E g I f\ 3 5 I I :3 58 I G I N J I J I 5 I I INVENTORS E E E z E RICHARD A HACKBORN WILLIAM W. MISSON JENS H. MO I ATTORNEY United States Patent 3,175,161 ELECTRGNIC CIRCUIT FOR ALTERING THE SLOPE 0F RAMP SIGNALS PRODUCED DURING TIE TIME INTERVALS BETWEEN RECURRING PAIRS 0F PULSES, THE SLOPE BEING ALTERED IN ACCORDANCE WITH THE TIME DIFFER- EN CE BETWEEN A PREVIOUS TIME INTERVAL, AND THE DURATION OF A CORRESPONDING RAMP SIGNAL Richard A. Haclrhorn, Palo Alto, and William W. Misson, Mountain View, Calif., and Jens H. Mo, Trondheim, Norway, assignors to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Filed Jan. 24, 1963, Ser. No. 253,656 7 (Ilaims. (Cl. 328-185) This invention relates to apparatus for graphically recording applied electrical signals.

Graphic recorders have been constructed which use sampling techniques to reproduce applied electrical signals as series of dots on a recording medium. One known recorder of this type uses a spark point or stylus which sweeps across a special spark-sensitive recording chart in synchronism with a sweep voltage. At the instant when the sweep voltage equals the applied signal voltage, a spark is produced between the stylus and the recording chart, which spark leaves a visible mark upon the chart. The applied signal is thus reproduced as a series of dots. A recorder of this type is described in the literature (See: Instrument for Simultan Registrering Av Flere Stprrelser by Jens G. Balchen og lens H. Mo; Teknisk Ukeblad 33;

15 Sept. 1955, pgs. 712-716).

One disadvantage inherent in this type of recorder is that slight changes in the stylus speed due, for example, to changes in line frequency cause an advance or a retardation of the position of the stylus with respect to its normal position as determined by the amplitude of the sweep voltage at a given instant in the sweep cycle. It is therefore desirable in recorders of this type to provide a sweep voltage which has a slope that varies in accordance with the speed of the stylus. The position of the stylus for a given sweep voltage is thus accurately determined at any instant in the sweep cycle, independently of the sweep speed of the stylus.

Accordingly, it is an object of the present invention to provide means for regulating the slope of a sweep voltage in accordance with the speed of a stylus.

It is another object of the present invention to provide improved means for regulating the slope of a sweep volt age.

It is still another object of the present invention to r provide means for identifying which one of a plurality of signal channels produces a given signal trace on the recording chart.

In accordance with the illustrated embodiment of the present invention a stylus is carried across the recording paper on a flexible belt connecting two rotating wheels. A pair of pulses are derived from the belt to mark the beginning and the end of the movement of the stylus across the chart. The pulse which marks the start of the movement of the stylus initiates a sweep voltage. This sweep voltage is compared with an applied signal to produce a marker pulse when the two signals are equal, which marker pulse lays down a dot on the chart at a distance from the start of the sweep which is related to the amplitude of the applied signal. A time-varying signal is initiated at the end of the sweep voltage. This time-varying signal is sampled in response to the pulse which marks the end of the movement of the stylus. The amplitude of this sample pulse is thus related to the time separation between the end of the sweep voltage and the end of the stylus sweep. A signal derived from the sample pulse is applied to the sweep-voltage generator to vary the slope 3,175,161 Patented Mar. 23, 1955 of the sweep voltage so that coincidence between the end of the sweep voltage and the end of the stylus sweep is maintained. The slope of the corrected sweep voltage thus bears the proper correspondence to the speed of the stylus in a subsequent sweep cycle to establish coincidence between the end of the sweep voltage and the end of the stylus sweep.

Other and incidental objects of the present invention will be apparent from a reading of this specification and an inspection of the accompanying drawing which shows a block diagram of the recorder and sweep synchronizing apparatus of the present invention.

Referring now to the drawing there is shown a pair of rotatable wheels 9 and 11 connected together by a flexible belt 13. The belt carries a plurality of styli 14, 15 and 16 which traverse the width of recording chart 17 disposed against the grounded drum 19. The belt 13 also carries a pair of apertures 21 and 23 for each of the styli. These apertures are so disposed on the belt as to identify the finish of the sweep of one stylus and the start of the sweep of the next stylus across the width of recording chart 17. An optical system including light source 25, the apertures 21 and 23 in the belt 13, and photoresponsive element 27 produces a pulse each time one of the apertures is aligned with the light path. The pulse thus produced is amplified by amplifier 29 and passes through the pulse separator and gate 31 either along line 33 to one input of ramp binary 35 or along line 37 to the sampling switch 39. The output of the ramp binary 35 is connected to switch 41 to the ramp generator 43, to the time-varying signal generator 45 of the ramp slope regulator 47 and to the pulse separator and gate 31. The output of ramp generator 43 is applied to an input of each of the amplitude comparators 49, 51, etc. and to one input of ramp comparator 53. The other input of ramp comparator 53 is connected to reference source 55 and the output of comparator 53 is connected to the other input of ramp binary 35. The outputs of the amplitude comparators 49, 51, etc. are combined on line 57 and are applied through the pulse shapcr 59 and pulse amplifier 61 to the wheel 9, and hence to the belt 13. The output of each of the amplitude comparators 49, 51, etc. is applied through a gate 63, 65, etc. to common line 67 connected to the input of indicator circuit 69. The pulse width control 71 of pulse shaper 59 is connected to receive the output of the indicator circuit 69. Each of the gates 63, 65, etc. is controlled from a contact on switch 73 which is carried by drum 19.

In operation, the wheels 9 and 11 are rotated by motor 12 at an angular velocity which remains substantially constant over a sweep cycle but which is subject to small variations with changes in line frequency. This carries each of the styli 14, 15, 16 on belt 13 in succession across the width of the recording chart 17 at a linear velocity which also remains substantially constant over the sweep cycle. The recording chart 17 is advanced at a predetermined rate by the chart-driving mechanism which includes motor 20, and the sprocket drive 18 on drum 19.

A sweep cycle is initiated at the instant when a stylus, say stylus 15, is aligned with one edge of the recording chart. At that instant a signal is produced by the photoresponsive element 27 in response to light from light source 25 which passes through the aperture 23 in the belt 13. This signal is amplified by the amplifier 29 and is applied through the pulse separator and gate 31 along the line 33 to the ramp binary 35. The ramp binary produces a signal on line 36 in response to the change in operating states caused by the signal on line 33. The signal appearing on line 36 at this time performs two functions; first, it opens switch 41 of ramp generator 43 and second, it changes the operating state of pulse separator and gate 31 so that the next pulse applied thereto from the element When switch 41 opens a sweep voltage having substantially linear slope is produced on line 42 by the combination of amplifier 44 and capacitor 46 operating as a Miller integrator upon the signal supplied through resistor 48 from supply terminal 56. In practice, this ramp voltage starts from a positive voltage and decays linearly toward a negative voltage. This ramp voltage is compared with an input voltage appearing at an input, say input N, by amplitude comparator 49. At the instant when these two voltages are equal a pulse is produced by amplitude comparator 49 which is shaped by pulse shaper 59, is amplified by the pulse amplifier 61 and is applied to stylus 15 through wheel 9 and belt 13. This pulse appearing on stylus 15 with respect to the grounded drum 19 produces a spark discharge through the recording chart 17, which spark discharge leaves a visible mark on the chart. Thus the amplitude of the signal appearing at input N when the sample is taken is characterized by the distance from the start of the sweep to the mark on the chart. The amplitudes of signals appearing at each of the remaining inputs N1, N-Z, etc. during a sweep cycle are also characterized by marks laid down in a similar manner on the recording chart 17 during the same stylus sweep. The several input channels may thus be separated and selectively located on any portion of the recording chart 17 simply by varying the average DC. voltage on the input signal of one channel relative to the average DC. voltage on the input signal of an adjacent channel. After many sweep cycles the input signal appearing on each of the input channels appears on the recording chart as a signal trace 77 composed of many dots or marks. The scheme for identifying which of the signal traces was produced by a given input signal will be described later.

At the same time that the ramp voltage on line 42 is applied to the amplitude comparators 49, 51, etc., it is also applied to the ramp comparator 53. When this ramp voltage and the voltage supplied by reference source 55 bear a predetermined relationship to each other a signal is produced at the output of the ramp comparator 53 which is applied to the input of ramp binary 35. This causes the ramp binary 35 to return to its initial operating state and thereby to produce an output signal on line 36, which output signal performs two functions. First, it closes switch 41 of ramp generator 43, thereby terminating the ramp voltage on line 42 and seconds, it renders the timevarying signal generator 45 operative, thereby producing a signal which increases in amplitude very rapidly with time. This time-varying signal may thus commence at a time either before or after the stylus 15 has traversed the width of the recording chart 17 depending upon whether the speed of the stylus has varied greatly from the speed during the previous sweep cycle. When the stylus has traversed the width of the recording chart 17 a pulse is produced by the photoresponsive element 27 in response to light from the light source 25 passing through an aperture 21 in belt 13. This pulse is amplified and passes through pulse separator and gate 31 along line 37 to the sampling switch 39. This causes a sample of very short duration to be taken of the time-varying signal produced by the generator 45. Since the time-varying signal is initiated at the end of the ramp voltage and since the sample pulse is produced at the end of the stylus sweep, the amplitude of the sample pulse bears a direct relationship to the time separation between the end of the ramp voltage and the end of the stylus sweep. Ideally, this time separation should be negligible to ensure that the linear displacement of the stylus along the width of the recording chart 17 is always related by a constant factor to the amplitude of the ramp voltage at any given instant, independent of the linear velocity of the stylus. Practical consideration necessitates allowing a small separation in the coincidence of these two events in order to provide ramp slope correction as described later. A fixed time 4 separation may be provided simply by displacing the location of aperture 21 with respect to the stylus.

Correction of the slope of the ramp voltage for a given sweep cycle is provided at the end of a previous sweep cycle. The sample pulse produced in response to the end of the previous stylus sweep occurs at a time when the time-varying signal has a certain value related to the time separation between the end of the ramp voltage and the end of the stylus sweep for that sweep cycle. This sample pulse is applied to the integrator 40 which produces a steady signal on its output related to the amplitude of the sample pulse. This steady signal in combination with the signal supplied through resistor 43 thus determine the slope of the ramp voltage on line 42 during the next sweep cycle. If the linear velocity of the stylus or if the slope of the ramp voltage varies from one sweep cycle to the next sweep cycle, then the pulse marking the end of the stylus sweep occurs earlier or later than normally. As a result, the amplitude of the sample pulse will vary about the normal amplitude and will change the amplitude of the steady signal at the output of the integrator. If the combination of this steady signal and the signal supplied through resistor 48 varies about the normal value then the slope of the ramp voltage produced during the next sweep cycle is varied in the proper direction to restore normal operating values. In order to provide correction of the slope above and below the normal value, it is desirable to select the value of the supply voltage on terminal 50 and the value of resistor 48 to provide approximately 90 percent of the signal normally required by generator 43 and to design integrator 40 to provide another 5 to 15 percent of the required signal. Overall correction of 5 percent in either direction is thus achieved. Since the inertia of the wheels 9 and 11 is large, the velocity of the stylus can vary only by small increments per sweep cycle. Also, since the operating conditions remain substantially unchanged from one sweep cycle to the next, the slope of the ramp voltage is virtually unaffected. Thus the relationship between the slope of the sweep voltage and the speed or" the stylus can, in practice, be maintained Well within .01 percent.

It is possible in the sampling-type recorder of the present invention to produce the signal traces 77 from each of the input channels on any portion of the width of the recording chart 17. Thus it is essential to provide suitable means for identifying which signal trace represents a given input signal. This is accomplished in the present invention by passing the output pulse produced by an amplitude comparator 49, 51, etc. through a gate 63, 65, etc. to a common line 67 connected at the input of the indicator circuit 59. These gates are enabled in timed sequence by the rotating contacts on switch 73 carried by the drum 19. Each time a gate is enabled the pulse which is produced by the amplitude comparator when the ramp voltage and the input voltage are equal appears both on lines 57 and 67. The indictor circuit 67 renders the pulse width control 71 effective to vary the time during which the pulse shaper 59 remains in one operating state. This may be accomplished, for example, by varying the voltage toward which an R-C circuit in a multivibrator charges. Thus if gate 63 is enabled, pulses appear on line 57 during a single sweep cycle each time the amplitude of an input signal bears a predetermined relationship to the ramp voltage on line 42. The pulse produced by the amplitude comparator 49 appears on line 57 as one of these pulses and in addition is the only one which appears on line 67 during the sweep cycle. This causes the pulse shaper 59 to increase the width of the pulse, which pulse width is determined by the time that the indicator circuit 69 remains in one operating state. The pulse produced by the amplitude comparator 49 in each of the sweep cycles which occur while gate 63 is enabled is thus identified as a pulse having wider width than the other pulses produced by the other amplitude comparators during the same sweep cycles. This cit-4 condition prevails until another gate is enabled by switch 73. The widened pulse 75 produces a broader mark on the signal trace in each sweep cycle. Thus, if gate 63 is enabled for a number of sweep cycles, a short segment 79 of the signal trace appears as a broad line. This broad line segment appears in line with a number on the recording chart which identifies the signal channel. A similar result is obtained by producing an oscillatory signal for the duration of the widened pulse in each sweep cycle. Such an oscillatory signal produces a number of sample marks in each sweep cycle, thus presenting the appearance of a broad line segment. The accuracy of the signal trace remains unaffected by the broad line segment since the width of the marks are increased from the reference edge of the signal trace, which reference edge is formed by the starts of both the normal and the broadened marks. Since these broad line segments identifying the signal channels are related only to the position of the switch 73 relative to the recording chart 17, proper identification of signal channels is insured by aligning a reference mark on the sprocket drive 18 characterizing a given channel with the number on the recording chart 17 identifying that channel.

Therefore the recording apparatus of the present invention maintains constant the relationship between the linear velocity of the stylus across the width of a recording chart and the slope of a ramp voltage with which an input signal is compared. The accuracy of the present recording apparatus is thus independent of the speed of the stylus and the drift of the ramp generator. In addition, since the slope of the sweep voltage is corrected prior to each successive sweep cycle high accuracy is obtained despite continually changing operating conditions. In addition, a recorder operated in this manner is capable of simultaneously producing a plurality of signal traces on a recording chart. Each signal trace thus produced may be selectively located over any portion of the width of the chart and may overlap the signal traces of adjacent channls. Further, identification of signal traces is automatically provided and is not affected by the position of the signal trace on the recording chart.

We claim:

1. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

means initiating a first time-varying signal in response to the start of the movement of said element over said selected distance; means initiating a second time-varying signal in response to the end of said first time-varying signal;

means to sample said second time-varying signal in response to the end of the movement of said element over said selected distance;

means producing a signal having an amplitude related to the amplitude of said sample; and

means responsive to the amplitude of said signal to alter the rate of change of said first time-varying signal during a successive recurrence of the move ment of said element over said selected distance.

2. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

means initiating a ramp signal in response to the start of the movement of said elment over said selected distance;

means initiating a time-varying signal in response to the end of said ramp signal;

means to sample said time-varying signal at the end of the movement of said element over said selected distance;

an integrator;

means including said integrator and producing a signal having an amplitude related to the amplitude of said sample; and

means responsive to the amplitude of said signal to alter the slope of said ramp during a successive recurrence of the movement of said element over said selected distance.

3. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

means producing start and stop signals respectively at the start and finish of the movement of said element over said selected distance;

means initiating a ramp signal in response to said start signal;

means initiating a time-varying signal in response to the end of said ramp signal;

means to sample said time-varying signal in response to said stop signal;

an integrator;

means including said integrator and producing a steady signal having an amplitude related to the integral of said sample; and

means responsive to the amplitude of said steady signal to alter the slope of said ramp during a successive recurrence of the movement of said element over said selected distance.

4. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

a circuit for producing start and stop signals respectively at the start and finish of the movement of said element over said selected distance;

a gate connected to receive said start and stop signals and adapted to produce a first output in response to a start signal and a second output in response to a stop signal;

means initiating a ramp signal in response to said first output;

means initiating a time-varying signal in response to the end of said ramp signal;

means to sample said time-varying signal in response to said second output;

an integrator;

means including said integrator and producing a steady signal having an amplitude related to the integral of said sample; and

means responsive to the amplitude of said steady signal to alter the slope of said ramp during a successive recurrence of the movement of said element over said selected distance.

5. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

a circuit for producing start and stop signals respectively at the start and finish of the movement of said element over said selected distance;

a gate connected to receive said start and stop signals and adapted to produce a first output in response to said start signal and a second output in response to a stop signal;

a trigger circuit adapted to operate only in either one of two operating states and being adapted to change operating states a first time in response to said first output applied thereto;

means initiating a ramp signal in response to the first change in the operating states of said trigger circuit;

means initiating a ramp-stop signal when said ramp signal attains a predetermined amplitude;

said trigger circuit being adapted to change operating states a second time in response to the ramp-stop signal applied thereto;

means producing a time-varying signal in response to the second change in the operating states of said trigger circuit;

means to sample said time-varying signal in response to said second output;

an integrator;

means including said integrator and producing a steady signal having an amplitude related to the integral of said sample; and

means responsive to the amplitude of said steady signal to alter the slope of said ramp during a successive recurrence of the movement of said element over said selected distance.

6. Electromechanical apparatus comprising:

an element adapted recurringly to move a selected distance;

means producing start and stop signals respectively at the start and finish of the movement of said element over said selected distance;

circuit means having a pair of outputs and being connected to receive said start and stop signals;

said circuit means producing a signal on said one output in response to said start signal;

a trigger circuit having an initial operating state and an actuated operating state;

said trigger circuit being connected to said one output and being adapted to change from the initial to the actuated operating state in response to signals applied thereto from said one output;

said circuit means being rendered effective by the operation of said trigger circuit in the actuated state to produce a signal on the other of said outputs in response to the stop signal applied thereto;

an utilization circuit connected to receive the signal on said other output; and

means to restore said trigger circuit to said initial operating state a selected time after the appearance of a signal on said one output.

7. Electromechanical apparatus comprising:

a plurality of rotatable Wheels connected by a belt;

means including said belt and producing start and stop signals respectively at the start and finish of the rotation of a Wheel through a selected angle;

circuit means having a pair of outputs and being connected to receive said start and stop signals;

said circuit means producing a signal on said one output in response to said start signal;

a trigger circuit having an initial operating state and an actuated operating state;

said trigger circuit being connected to said one output and being adapted to change from the initial to the actuated operating state in response to signals applied thereto from said one output;

said circuit means being rendered effective by the operation of said trigger circuit in the actuated state to produce a signal on the other of said outputs in response to the stop signal applied thereto;

an utilization circuit connected to receive the signal on said other output; and

means to restore said trigger circuit to said initial operating state a selected time after the appearance of a signal on said one output.

References Cited in the file of this patent UNITED STATES PATENTS Eisler et al Mar. 10, 1959 

1. ELECTROMECHANICAL APPARATUS COMPRISING: AN ELEMENT ADAPTED RECURRINGLY TO MOVE A SELECTED DISTANCE; MEANS INITIATING A FIRST TIME-VARYING SIGNAL IN RESPONSE TO THE START OF THE MOVEMENT OF SAID ELEMENT OVER SAID SELECTED DISTANCE; MEANS INITIATING A SECOND TIME-VARYING SIGNAL IN RESPONSE TO THE END OF SAID FIRST TIME-VARYING SIGNAL; MEANS TO SAMPLE SAID SECOND TIME-VARYING SIGNAL IN RESPONSE TO THE END OF THE MOVEMENT OF SAID ELEMENT OVER SAID SELECTED DISTANCE; MEANS PRODUCING A SIGNAL HAVING AN AMPLITUDE RELATED TO THE AMPLITUDE OF SAID SAMPLE; AND MEANS RESPONSIVE TO THE AMPLITUDE OF SAID SIGNAL TO ALTER THE RATE OF CHANGE OF SAID FIRST TIME-VARYING SIGNAL DURING A SUCCESSIVE RECURRENCE OF THE MOVEMENT OF SAID ELEMENT OVER SAID SELECTED DISTANCE. 